Catalysis
Catalysis provides an additional mechanism by which reactants
can be converted to products. The alternative mechanism has a
lower activation energy than the reaction in the absence of a
catalyst.
v0
A
B
v0 no catalyst
vc -- catalyst present
vc
(v0 = -d[A]/dt with no catalyst)
(vc = -d[A]/dt with a catalyst)
Potential Energy
E Not affected
by catalyst
Ea,f
Energy barrier
with catalyst
Energy barrier without catalyst
Ea,f
Ea,f and Ea,r
are lowered
by
catalyst
Ea,
f
Ea,r
∆E
Ea,r
Reactants
Reaction coordinate
Products
Generally a catalyst is defined as a substance which increases the
rate of a reaction without itself being changed at the end of the
reaction.
This is strictly speaking not a good definition because some things
catalyze themselves, but we will use this definition for now.
Catalyst supplies a reaction path which has a lower activation
energy than the reaction in the absence of a catalyst.
Catalysis by Enzymes
Enzymes may be loosely defined as catalysts for biological systems.
They increase the rate of reactions involving biologically important
systems.
Enzymes are remarkable as catalysts because they are usually
amazingly specific (work only for a particular
kind of reaction.)
They are also generally very efficient, achieving substantial
Rate increases at concentrations as low as 10-8 M!
Typical enzyme molecular weights are 104-106 gm/mole
(protein molecules)
Summary of Enzyme Characteristics
1) Proteins of large to moderate weight 104 - 106.
2) Extremely efficient (work at 10-8 M)
3) Very specific (work only on special types of reactions).
General Behavior of Enzyme Catalyzed Reactions
S (substrate)


Products
E
If the initial rate of the reaction is plotted versus the initial
concentration of substrate S for a constant enzyme concentration,
the following behavior is found:
Vi
Initial
Rate
[S] concentration when Vi = VS / 2
Substrate Concentration
Maximum initial rate
Vi
Initial
Rate
(Half maximum initial rate)
[S] concentration when Vi = VS / 2
Substrate Concentration
Vs is found to be directly proportional to the total enzyme
concentration (Eo): Vs ~ (Eo)
(S) concentration required to reach half maximum initial velocity
(Vi = VS/2) found to be independent of (Eo). (S)1/2 = KM
Explanation: Michaelis-Menten Mechanism
E is free enzyme and ES is an enzyme-substrate complex
It may generally be assumed that (S) >> (E) since E are so efficient
they catalyze reaction at very small concentration.
(ES )
= k1 (E) (S) - k-1 (ES) - k2 (ES)
d
dt Step 1
Step 2 Step 3
First Order Process
First Order
d(ES) Process
0
dt
Steady State assumption:
Second Order Process
k1 (E0 )(S)
= k1E0/[k1+(k-1+k2)/(S)]
(ES ) 
k 1  k 2  k1 (S)
Mechanism
k1
E + S  ES
Step 1
k-1
ES  E + S Step 2
k2
ES  P + E
Step 3
All are elementary kinetic steps.
Divide top and bottom by k1 to get 
Bonus * Bonus * Bonus * Bonus * Bonus * Bonus
k2
k2 (E0 )
(E 0 ) 
Km
2
1
(S) 1
VS/2
2
Km
 1
Case III :
(S)
(S)1/2 is the substrate concentration when the
initial rate reaches half its maximum value.
dP k2 (E0 )

dt 1  K m
(S)
Depends linearly on [S] in region of low substrate concentration.
dP
Vmax
V
Km
dt
1
(S)
Invert this
equation to
get---
Mechanism
k1
E + S  ES
Step 1
k-1
ES  E + S Step 2
k2
ES  P + E
Step 3
{Lineweaver-Burke Plot}
Km is rate at which ES decomposes by two mechanisms (k-1 or k2)
divided by rate constant for formation of ES.
Large Km  weak binding of E to S
Small Km  strong binding of E to S
k 1  k 2
Km =
k1
QuickTime™ and a
Animation decompressor
are needed to see this picture.
Application of chemical kinetics to ecological
and toxicological problems
I. Application of enzyme kinetics:
 Degradation of organophosphate pesticides
Enzymes can be used to catalyze degradation of pesticides
While extremely beneficial for protection of crops,
pesticides can have serious environmental impact
Possible deleterious consequences include seepage of
these otherwise helpful chemicals into soil and ground water
Case study: organophosphate pesticides
 abundant
 highly toxic
 “neutralized” via reaction
The same enzyme catalysts which can neutralize these pesticides
can also be used to detoxify chemical nerve agents.
 nerve agents are structurally similar to
organophosphate pesticides
Nerve gas used
General
in Japanese
formula for
subway attack.
pesticides
 both contain organophosphate esters
Degradation
product produced
in body from parathion
paraoxon
Recent studies have shown that enzymes, which effectively degrade
organophosphates, can be incorporated into polymers -specifically, foams -- in order to aid in their practical application.
One enzyme under current investigation is organophosphorus
hydrolase (OPH) a.k.a. phosphotriesterase.
Attack by water
[derived from Escherichia coli]
The degradation reaction of organophosphates works via hydrolysis.
(yellow in solution)